Electron holography has become a desirable technique for inspection of semiconductor devices. The principles of electron holography are described, for example, in U.S. Published Patent Application 2002/0084412 entitled “Electron Microscope Equipped with Electron Biprism,” incorporated herein by reference, and U.S. Pat. No. 5,192,867 entitled “Electron Optical Measurement Apparatus”.
The requirements for use of electron holography to inspect semiconductor devices are (1) an overlap of the plane wave transmitted through the specimen and the plane wave not transmitted through the specimen in the range of about 50 to 1000 nanometers, (2) fringe spacing between about 1 and 10 nanometers, and (3) adjustability of both the overlap and the fringe spacing. The overlap is roughly equivalent to the field of view. Spatial resolution is approximately twice the fringe spacing. Overlap and fringe spacing can be related. With some prior methods in which only one objective lens was used, if the overlap was satisfactory, the fringe spacing was unsatisfactory. Multiple lens systems have been proposed. A number of methods of using multiple lens systems are possible.
Applicants have discovered a manner of using a two-lens system that provides a satisfactory magnification and fringe spacing for two-dimensional (2D) dopant profiling of semiconductor devices. The method also provides a continuously varying low magnification to high magnification electron holography. The application of this method is not limited to semiconductor 2-D dopant profiling.